Apparatus for heating finely divided solid material



June '18, 1957 R. J. PLASS 2,796,249

APPARATUS FOR HEATING FINELY DIVIDED SOLID MATERIAL Filed April 5, 1955.ZTWENTaQ.

passer CZ Hess,

United States Patent APPARATUS FOR HEATING FINELY DIVIDED SOLID MATERIALRobert J. Plass, Los Angeles, Calif;, assignor to Western PrecipitationCorporation, Los Angeles, Calif., a corporation of CaliforniaApplication April 5, 1955, Serial No. 499,312

1 Claim. (Cl. 263-32) This invention rel-ates generally to a method andmeans for heating finely divided solid material; and more especially,for heating the raw feed prior to introduction into a rotary kiln in themanufacture of cement.

In a process of this character, the most effective and quickest, way inwhich the finely divided material can be heated is by direct contactwith the heated gases leaving the kiln. Recognizing this, various typesof apparatus have been devised in which the raw feed is introduceddirectly into the stream of gases from the kiln and is suspended in orcarried along by the gas stream. This then creates the problem ofeffecting a substantially complete or highly efiicient separation of thesolid and gaseous phases of the mixture.

Known arrangements for this purpose have generally included a pluralityof serially connected collectors of the cyclone type. These collectorseffect successive collections of the suspended material, which, aftercollection is introduced into the kiln. These known arrangements havevarious disadvantages, as for example the extensive and complicated ductwork required to convey the gas stream carrying the material insuspension from one collector to another and to convey the collectedportions back to the kiln. Such piping may include sharp bends or otherplaces where the drop in gas velocity causes an excessive accumulationof dust particles. Also, the piping arrangements are often expensive andtake up an excessive amount of space; or, because of the great length ofduct work through which the particles are conducted, they may cause anexcessive amount of heat loss.

Thus it becomes a general object of my invention to devise a method ofheating finely divided material by direct contact with heated gases andsubsequently separating' the suspended material in a plurality ofcollectors in such a way as to eifect maximum heat transfer to thefinely divided material and minimum loss of heat before ultimateintroduction of the material into the kiln in order to maintain the kilnfeed at a desired high temperature.

It is an object of the invention to provide apparatus for carrying outthis method, which apparatus is relatively simple and enables short,direct connections between the various parts, but without sacrificingcollection efficiencyf A further object of my invention is to provideapparatus for carrying out my improved method in which one or morefractions of material collected at a relatively low temperature can berecirculated and reheated in order to raise the temperature of theultimate feed to the The above objects of my invention are achievedaccording'to my improved method by introducing the feed in the form offinely divided solids into a rising stream of heated gas taken from thehead end of the kiln, the finely divided material being heated by directcontact with the gas stream as it is carried upward in suspension by thegas stream. After the desired time of contact, a first partialseparationof the heated material is effected by passingthe material through ,adust collector from which the collected material is introduced directlyinto the head end of the kiln, The gas stream with the remainingmaterial in suspension is then subjected to a second separation in whicha second and smaller fraction of the suspended particles is removed fromthe gas stream, this generally being composed of finer particles -sincethe larger part of the coarser particles has already been removed. Thissecond fraction is then added to and mixed with the unheated feed toraise the temperatures of the feed by direct contact with it prior tothe introduction of the feed into the heated gas stream.

My improved method is carried out in a rotary kiln which is connected atits head and to a vertically extending stack in which the heated gasesfrom the kiln rise. Feed means, preferably in the form of a screwconveyor or the like, introduces the finely divided solid material intothe stack at some suitable point in the lower portion of the stack inorder to obtain a satisfactory length of contact time between the hotgases and the feed as they are carried upward in the stack by the risinggas stream. Two cyclone separators are arranged side by side, the gasstream being passed first through one and then through the other. In thefirst collector a major portion of the suspended material is removedfrom the gas stream and this portion of the heated material is thenallowed to pass by gravity through a conduit which introduces itdirectly into the kiln. A smaller portion of the material is collectedin the second collector and then allowed to pass by gravity through aconduit which is connected to the feed means. This material, whileheated, is not hot enough for introduction into the kiln, so it is addedto the raw feed at some point ahead of the location of the discharge ofthe feed into the stack. Preferably a certain amount of mixing of thefeed takes place during the conveying in order that the preheatedmaterial may be thoroughly mixed with the raw feed and raise the averagetemperature of all the particles in the feed by the time the feed isintroduced into the stack.

A suitable blower is provided through which the gas stream passes, theblower providing the energy to force the gas stream through the twocollectors. It is normally located so that the blower inlet is connectedto the outlet of the second cyclone collector and the gas passingthrough the blower is not only at a lower temperature but also carriesonly a small amount of suspended material.

In order to effect a high degree of total collection, it is desirablethat the outlet of the blower discharge into an electrical precipitatorwhich is particularly well adapted to remove substantially .all of thevery fine particles remaining suspended in the gas stream. From theprecipitator, the cleaned gases may be discharged from the atmospherewhile the material collected in the precipitator is returned to the rawfeed.

How the above objects and advantages of my invention, as well as othersnot specifically mentioned, are attained will be more readily understoodby reference to the following description and to the annexed drawing, inwhich there is shown schematically an apparatus em-- bodying myinvention for heating finely divided material by direct contact withheated gases and then subsequently separating the heated material fromthe gas stream in two or more fractions which are either introduceddirectly into the kiln or mixed with the raw feed.

Referring now to the drawing, there is indicated genorally at 10 theupper end of a rotary kiln which may be of any conventional design. Theopen upper or head end of the kiln is surrounded by a stationary housing12. The housing 12 is connected to the lower end of vertically extendingstack or flue 14 which receives the heated gases discharged from thekiln. Stack 14 in conventional pracice discharges the heated gases intothe open atmosphere;

but according to this arrangement advantage is taken of the heatcontained in the gases within the stack to heat the feed material beforeentering the kiln. In accordance with the usual practice, stack 14 risesvertically;

w A first dust separator or collector 16 and a second dust collector 18are provided at positions suitably elevated above the level of kiln 10.These two collectors are preferably arranged in side-by-siderelationship and are horizontally spaced apart only a short distance, toreduce the length of connecting duct work and heat losses therefrom. Forthis same reason they are close to, but not necessarily directly overthe kiln. Each of the two collectors is of the cyclone type, preferablyof the type having a plurality of small cyclone tubes operating inparallel, since this type of collector has no moving parts and is wellable to withstand the relatively high temperatures of the gases flowingthrough them.

Each of the two collectors 16 and 13 is provided with a gas inlet and agas outlet at opposite sides and with a material outlet at its bottom.The upper end of stack 14 is connected to the gas inlet of the firstcollector 16 while the gas outlet of collector 16 is connected by ashort duct 20 to the gas inlet of the second collector 18. In this waythe gas stream after passing through stack 14 passes in successionthrough the two dust collectors.

Upon leaving the second dust collector 18, the gas stream passes throughduct 21 into blower means 22, which may be any type of air movingdevice, such as a centrifugal fan. The outlet or discharge from blowermeans 22 is connected to electrical precipitator 23 so that the gas uponleaving the blower enters the electrical precipitator. The gas outlet 24of the electrical precipitator may be open to the atmosphere or beconnected to any desired piece of apparatus. Material collected in theprecipitator is very fine and may be returned to the system at anydesired position, suitable conveying means, either power or gravity,being indicated diagrammatically at 27.

At the lower end of collector 16 there is a material outlet opening tosealing valve 26 through which the material collected in the collectoris discharged into duct 25. Valve 26 prevents flow of gases past thevalve and through duct 25. Duct is preferably vertical or steeplyinclined, and is connected to feed pipe 28 which corresponds to theconventional means for introducing feed into the head of kiln 10. Thisfeed pipe passes through a portion of housing 12 and into the open endof kiln 10 to discharge material by gravity into the kiln.

The materials to be fed into the kiln are ground or pulverized to thedesired degree of fineness by machinery not shown in the drawing. Thisraw feed, in finely divided form, is then brought by suitable means,indicated diagrammatically at 29, to the feed or supply means as shownin the drawing. This means consists of a hopper 30 which is connected atits lower end to screw conveyor 31 or other suitable positive conveyingmeans, which extends preferably in a generally horizontal direction.Near one end screw conveyor 31 discharges the finely divided materialthrough a discharge pipe 32 into stack 14. Discharge pipe 32 is locatedin the lower portion of stack 14. Obviously it is desirable that thematerial be introduced into the stack at as low a position as possible,but at the same time it is preferable that the point of entry be at alocation at which all of the gas within the stack is moving upwardly.

At the lower end of the second collector 18 is a material outlet whichis connected to scaling valve 34. This valve is also connected to thevertically extending conduit 35. By operation of valve 34- materialcollected within the hopper is transferred to conduit 35 and then fallsby gravity through the conduit, the lower end of which is connected tothe housing of screw conveyor 31.

Having described the apparatus, I shall now describe the method involvedand the operation of the apparatus. Prior to introductio into hopper 30,the component parts of the raw feed are ground to the desired degree offineness and mixed together in the proper proportions; and this mixtureis finely divided, dry solid material is then conveyed by means 2% tohopper 30 from which it flows by gravity to the screw conveyor or othermeans for supplying the raw feed to the system. The flow of the materialis indicated by the arrows. The flights of the screwconveyor as theyrotated advance the powdered material to the right and finally introduceit through pipe 32 into stack 14. The material has been ground to such adegree of fineness that it is substantially all carried upward in thestream of hot gases rising within stack 14. Being in dry form, thepulverized material is easily dispersed throughout the gas stream by theturbulence of the gases.

Being in direct contact with the heated gas, the particles of materialare heated quickly and efiiciently; and by the time the top of the stackis reached, these particles have reached a temperature of approximatingthat of the gas stream in which they are suspended. During this upwardtravel, of course the gas has dropped considerably in tem-- perature andtherefore decreased in volume. For example, gas leaves the kiln attemperature usually within the range of 1300-l700 F. and are cooled toabout 8001100 F. at the inlet to the first collector. In order tocompensate for this volume change and keep the gas velocity throughoutthe stack at a substantially constant value, the stack tapers upwardly.The diameter or cross sectional area of the stack decreases in an upwarddirection and at a rate designed to substantially compensate for thedecrease in volume of the gases.

By the time the gas and suspended particles reach the top of the stack,the particles are heated to the temperature of the gas. At the upper endof the stack, the gas stream and the suspended particles are introducedinto the inlet of the first collector 16 in which a first partialseparation of the heated suspended particles from the gas is effected.In this collector, a major proportion of the suspended material, forexample 70% or more, is removed from the gas stream and collected in thehopper at the bottom of the collector. Coarser particles are more easilyseparated than finer ones, and therefore this first fraction includesmost of the coarser particles. It therefore may be considered as being acoarser fraction as compared with any fraction subsequently collected.This first fraction is then passed through sealing valve 26 into pipe 25through which it flows by gravity to feed pipe 28 through which thismaterial is introduced directly into the head end of rotary kiln 10. Atthe time of introduction, the feed is typically at about 600 -l000 F.

After leaving first collector 16, the gas stream with the remainingparticles in suspension is introduced into the inlet of the secondcollector 18 within which a second partial separation of the suspendedmaterial is efiected. Since nearly all of the coarser particles hadpreviously been removed, the second fraction collected in collector 13contains a larger percentage of finer particles and therefore may bereferred to as the finer fraction. fraction of the material is collectedin the bottom of collector and passed through sealing valve 34 intoconduit 35 through which the material flows by gravity. This secondfraction is combined with the raw feed being moved by the screwconveyor. At the point of introducing material from conduit 35, the rawfeed is unheated while the material introduced from conduit 35 is at ahigh er temperature. For example, the material introduced at this pointmay be within the range of 400800 F.

This second fraction of material is mixed with the unheated raw feed bythe mixing action of the flights of the screw conveyor so that as thefeed advances, all of the feed is heated to some extent. For example theaverage temperature of the material leaving the discharge 32 is in therange -200 F. Conduit 35 is preferably spaced sufficiently far fromdischarge 32 to permit a thorough mixing and heat transfer betweenparticles of This the mix as it is conveyed from the junction withconduit 35 to the point of discharge 32.

In order to keep up the velocity of the gas stream and to compensate forthe draft loss in passing through the two collectors, suitable blowermeans are provided for forcing the gas stream through the collectors.This blower means may take the form of any type of fan or blower,although a fan of the centrifugal type is shown in the drawing. Theinlet of the fan is connected by duct 21 to the outlet of centralcollector 18; and the gas stream leaving the vent is discharged intoelectrical precipitator 23. The fan may be located at any other desiredposition in the circuit; but the location shown has the advantages thatat this point the gas stream is relatively clean and the volume issmaller than at any point in the system between the blower and the kiln.Gas ten1perature at the intake to the blower is typically about 400- 600F.

The gas stream by the time it reaches precipitator 23, is carrying inSuspension less than of the original finely divided material; but theseparticles are the finest ones and therefore the hardest to collect.Because of the relatively high efficiency of the electricalprecipitator, normally in excess of 95%, this type of collector is thelast one of the series. It is also used in this position in the circuitbecause here the dust loading in terms of unit weight per unit volume isthe lowest.

The finely divided material collected from the electric precipitatorhas, by the time it leaves the precipitator, cooled down to atemperature only silghtly above ambient temperature. Hence this materialmay either be returned to the system at hopper 30 as shown or it may becombined with the raw feed prior to proportioning and returned throughconveyor 29.

Since changes in my invention may occur to those skilled in the artwithout departing from the spirit and scope of my invention, the abovedescription is considered as not being necessarily limitative upon, butrather illustrative of, the appended claim.

I claim:

The combination with a rotary kiln, comprising: a vertically extendingstack connected to one end of the kiln to receive heated gas therefrom:positive feed means for introducing solid material in pulverized forminto the lower portion of the stack to become suspended in the upwardlymoving gas stream; a first cyclone-type collector located above the kilnwith its inlet connected to the top of the stack to receive the gasstream and suspended dust, the material outlet of said separator beingconnected to the kiln to discharge collected material by gravity intothe kiln; a second cyclone-type collector located above the kiln withits gas inlet connected to the gas outlet of the first collector toreceive the gas stream, the material outlet of the second collectorbeing connected to the feed means at a location ahead of the dischargeinto the stack to discharge by gravity into the feed means materialcollected by the second collector; blower means connected to the gasoutlet of the second collector and through which the gas stream passesto force the gas stream through said collectors; an electricalprecipitator above the feed means with its inlet connected to the outletof the blower means to receive the gas stream; and conduit meansconnecting the material outlet of the precipitator to the feed means ata position ahead of the connection of the second collector to the feedmeans to discharge by gravity into the feed means the material collectedby the precipitator.

References Cited in the file of this patent UNITED STATES PATENTS2,290,068 Peterson July 14, 1942 2,356,717 Williams Aug. 22, 19442,648,532 Muller et a1. Aug. 11, 1953 2,719,034 Peterson Sept. 27, 1955FOREIGN PATENTS 1,043,069 France Nov. 5, 1953

